Abstract

Ab initio studies have been carried out on glycyl-L-alanine zwitterion using the basis sets 6-31 + g(d), 6-31++g(d,p) and 6-311g(d,p) to obtain its stable molecular conformation both in the gas phase and in solvent media of dielectric constants \varepsilon = 2.2 and 78.0 corresponding to carbon tetrachloride and water, respectively. The harmonic vibrational frequencies and their infrared and Raman band intensities of glycyl-L-alanine zwitterion have been calculated and their assignments are discussed. The solvent effects on the conformation and vibrational spectra of glycyl-L-alanine zwitterion in solvents of \varepsilon = 2.2 and 78 have been investigated by the self-consistent reaction field theory. The interaction of a dielectric medium has only a marginal effect on the molecular conformation and vibrational spectra. However, the results are in accord with the increasing weight of the covalent resonance structure over the dipolar structure for the amide group in polar solvents.